The present invention generally pertains to audio signal processing and is particularly directed to improved companding of digital audio signals.
Digital audio signals are companded to conserve transmission bandwidth. A prior art system used for companding digital audio signals for transmission with, and during the horizontal blanking interval of, television signals is described in U.S. Pat. No. 4,608,456 to Woo H. Paik, Jerrold A. Heller and Gordon Kent Walker. In the transmitter of the system described therein, an analog audio signal is converted into digital audio signal samples. Each digital audio signal sample is compressed into a combination of a gain word (referred to therein as an "exponent", a plurality of magnitude bits (referred to therein as a "mantissa") and a sign bit. In the compression process, the most significant bits of the digital audio signal sample are shifted in position in accordance with the value of the gain word; and the other bits are truncated. The compressed digital audio signal sample is forward-error-correction coded by a Hamming code generator, which generates code bits for detecting and correcting single-bit errors in a combination of the sign bit, the gain word and the magnitude bits; and is further forward-error-corrected by a parity bit generator, which generates a parity bit for detecting double-bit errors in a combination of the most significant magnitude bits and/or the parity bit. At the receiver, detected single-bit errors in the transmitted error-coded compressed digital audio signal samples are corrected; and detected double-bit errors are concealed by repeating the last-received correct or corrected sample. The received compressed samples are expanded at the receiver by a process of shifting the position of the received magnitude bits in accordance with the value of the gain word and by appending bits of a nominal value in the unfilled bit positions remaining after such shift to reproduce the digital audio signal samples. The reproduced digital audio signal samples are converted to a reproduced analog audio signal. By virtue of the truncation and appending steps in the respective compression and expansion processes, errors are inevitably induced in the reproduced digital audio signal samples.